1. Field of the Invention
The present invention relates to a liquid lens containing two different liquids therein, and more particularly, to a liquid lens which has a hole for housing the liquids perforated to a minimal depth to achieve slimness and compactness.
2. Description of the Related Art
A lens module in general use employs a lens made of solid such as glass whose focal distance is fixed. To perform focusing, such a lens module adopts a plurality of solid lenses which should be adjustably distanced from each other. This restricts miniaturization of the product and complicates driving for focusing.
For the purpose of overcoming the problem, a liquid lens has recently emerged. That is, the lens has two different liquids contained therein and is adapted to adjust its focus by changing the curvature radius of the interface (or meniscus) between the liquids. The liquid lens features a simple internal configuration which ensures miniaturization, and performs focusing more easily.
In early 2000, studies oh the liquid lens using electrowetting were conducted with tangible results, producing various applications. Also, efforts to put such liquid lens into industrial use have been stepped up. The liquid lens is advantageous over the conventional solid lens in terms of smaller size, low power consumption, fast response speed, and excellent reproducibility. Therefore, there have lately been vigorous attempts to apply the liquid lens to an automatic focus module of cameras mounted in mobile phones and PDAs.
A conventional liquid lens will be explained in detail with reference to the accompanying drawings.
FIG. 1 is a cross-sectional view illustrating a conventional liquid lens according to a first embodiment of the invention. FIG. 2 is a cross-sectional view illustrating a conventional liquid lens according to a second embodiment of the invention. FIG. 3 is a cross-sectional view illustrating a conventional liquid lens according to a third embodiment of the invention.
As shown in FIG. 1, the conventional liquid lens includes a chamber 10, a transparent plate 20, and two liquids. The chamber 10 is provided with a hole having a spherical cross-section. The transparent plate 20 hermetically seals an underside surface of the hole 12. Also, the two non-miscible liquids are housed in the hole 12.
Here, an insulating film is applied onto an inner wall of the hole 12 to ensure operation of the liquid lens.
Moreover, the two types of liquids are substantially equal in weight but different in refractivity. One of the liquids is conductive and the other liquid is non-conductive. In general, the liquid located at a top side is a conductive electrolytic fluid A and the liquid at a bottom side is a non-conductive insulating fluid B.
The two fluids A and B, when housed in the hole 12, have a predetermined degree of initial contact angle θ owing to different affinities of the fluids A and B to a wall of the hole 12, thereby forming a predetermined interface shape. The electrolytic fluid A and the insulating fluid B cooperate with the insulating film to form a concave interface between the fluids A and B.
The liquid lens configured as just described is subject to electrowetting when a voltage is applied and has an acute final contact angle. That is, the electrolytic fluid A and the insulating fluid B have an upwardly convex interface so as to function as a lens.
An ideal height of the liquid lens, when manufacture margin or environmental factor are not considered, is determined by an initial contact angle of the liquid (a contact angle when zero voltage is applied), a final contact angle (a contact angle at a highest operational voltage) and a shape of the hole 12. The electrolytic and insulating fluids A and B should possess lens characteristics to be employed in the liquid lens. Thus, the hole 12 is filled with pure electrolytic and insulating fluids A and B without any foreign elements such as bubbles that obstruct transmission of light, and then hermetically sealed. Accordingly, the height of the liquid lens is varied by that of the interface between the electrolytic and insulating fluids A and B.
When a contact angle is determined by the electrolytic fluid A, insulating fluid B and the inner wall of the hole, the interface between the electrolytic fluid A and the insulating fluid B (hereinafter “interface”) becomes partially spherical. In a case where the interface has an acute contact angle due to electrowetting, the interface also becomes partially spherical.
Here, the initial contact angle is a value peculiar to characteristics of the liquid and thus varies with the type of the liquid housed in the hole 12. For example, in a case where the hole 12 is filled with the insulating fluid B having a contact angle of δ, which is bigger than θ, the interface, as shown in FIG. 2, is more concave. In this fashion, the greater initial contact angle of the interface leads to more concave interface, thereby necessitating a greater depth of the hole 12. This accordingly increases an overall height of the liquid lens, hampering miniaturization of the product.
To manufacture the chamber 10 for the liquid lens, the hole 12 has a minimum height determined by the initial contact angle of the liquid and a shape thereof. Then, the height of the hole 12 is optimized in consideration of a safety factor to design a lens with a smaller depth.
Also, in a case where an undercut is formed in a lower part of the inner wall of the hole 12 to facilitate manufacturing, when the interface is changed to form a final contact angle, a contact point of the interface and the inner wall of the hole 12 may be located lower than the undercut, thereby disabling the lens function.
Especially, in a case where the inner wall of the hole 12 is inclined inward so that it narrows downward as shown in FIG. 3 in order to obtain a lens with a smaller depth, a height from edge portions to a mid portion of the interface differs from that of the first conventional example shown in FIG. 1 despite the identical contact angle. Here, the difference of the height is varied according to change in an inclination angle of the inner wall of the hole 12. As described above, the minimum height of the hole 12 is optimized depending on the contact angle of the interface and shape of the inner wall to design the liquid lens effectively.